Your questions all relate to the safety of pesticide residues that may occur in GM crops.
That’s a reasonable concern given the rapid adoption and widespread use of GM crops. Importantly, since crops tolerant to herbicides such as glyphosate are very popular among farmers, spraying of glyphosate could lead to residues of the active ingredient in the forage or grain that is consumed by animals or humans. When farmers spray fields to eliminate weeds that compete with the crop and reduce yield, the vast majority of the glyphosate enters plants through the leaves. Glyphosate is tightly bound to soil, and little or no glyphosate is taken up from the soil, either by newly planted seeds or by existing plants, whether GM or non-GM. One of the reasons that glyphosate is so popular with farmers is that farmers can safely plant other crops after using glyphosate without impacts on the subsequent crop. Over time, soil microorganisms break down any glyphosate residues in the soil.
Any glyphosate residues that remain in the plant decrease over time following application, and are less in grain compared to leaves. Processing of grain for use in food also reduces detectible residues. For example, there is no detectible glyphosate present in the oil fraction in soybean or corn oil.
Finally, since there is the potential for residues of glyphosate to remain in forage and grain used in animal feed and human foods, the levels must be measured across many locations and environments to determine the highest levels that might be present. In the US, the EPA is responsible to examine all uses of pesticides and must examine the residue data and establish safe levels of exposure. All uses must be approved and the combined exposure from all crops must be below the acceptable dose level established by the EPA. This process was described previously in detail on this site. That answer can be found at: (http://gmoanswers.com/ask/how-are-gmo-foods-regulated). Other countries follow similar procedures within their regulatory agencies.